On manufacturing of circuits in semiconductor devices, certain deposition or etch steps performed in ionized gases use electron sources. Indeed, it may for example appear to be necessary to negatively bias the surface of a substrate by bombarding it with an electron beam. An electron source may further be necessary to generate a plasma, to increase the ionization rate of a plasma, or yet to accelerate the ions of a plasma in which a substrate is placed. Further, different electron beam powers are desired to be used according to the considered application. For example, it may be necessary to have an electron beam with a power on the order of 100 eV to enable erosion (etch) of a substrate.
Electron sources of point type such as a heated emissive cathode are already available. However, such sources have the disadvantage of providing beams with a very small cross-section. The maximum possible processing surface area is thus very limited. Further, such sources can seldom be used in the considered applications, since the involved gases (plasma) risk reacting with the electrode.
Part 5.5.2 “Electron beam characteristics of miniaturized ECR plasma cathodes” of work “Electron Cyclotron Resonance Ion Sources and ECR Plasmas” by R. Geller, published by the Institute of Physics Publishing Bristol and Philadelphia (pages 352-353) describes electron sources in which electrons are extracted from a plasma. However, such sources cannot be sources of large surface area. Indeed, to be able to increase the electron extraction surface area of a plasma, the plasma volume has to be increased. Then, obtaining the electromagnetic field necessary to excite the plasma at the cyclotron resonance imposes generation of a continuous axial magnetic field of excessively high intensity. The generation of such a magnetic field, intended to confine the trajectory of the extracted electrons around the magnetic field lines imposes using particularly complex, bulky, and expensive installations. Further, the presence of a continuous axial magnetic field may be a problem in certain applications. This limits the extraction surface area to a few square millimeters. Further, electron sources of this type generate electron beams of an intensity limited to the order of one hundred milliamperes. Further, the current density exhibits an exponential-type variation along with the beam extraction power. It is thus impossible to extract on an extended surface area an electron beam of a current density which remains relatively steady when the beam extraction potential is modified. Further, said potential is at most on the order of some hundred volts.
The present invention aims at providing an electron source which exhibits at least some of the following features:                usable for different application fields,        capable of providing a beam, with as high as possible an electron current density,        relatively steady when the beam power is varied,        beam power controllable within a wide range,        surface area as extended as possible, for example, with a diameter from a few centimeters to a few tens of centimeters, or more,        free of the problems of known sources, and especially, exhibiting no axial magnetic field at the level or downstream of the extraction.        